Abstract With increasing concern over the threat of a nuclear attack or radiation bioterrorism, there is an urgent need to develop medical countermeasures against the acute radiation syndrome. Although a number of drugs can prevent radiation injury when delivered before radiation or improve survival when administered a few hours after radiation, these agents are not practical for a mass casualty situation. In this scenario, it may take at least 24 hours to identify and treat civilians exposed to a potentially lethal dose of radiation. To address this unmet need, we have identified glycogen synthase kinase-3 as a target, which can be inhibited 24 hours after a lethal dose of total-body irradiation, to improve survival from the acute radiation syndrome. In this proposal, we will fully characterize inhibitors of glycogen synthase kinase-3 that function as mitigators of the acute radiation syndrome when administered 24 hours after irradiation, identify early surrogate biomarkers for efficacy, and utilize mouse genetics to define mechanisms of action. The overall goal of this project is to develop a glycogen synthase kinase-3 inhibitor into a countermeasure against radiation when administered at least 24 hours after radiation.